Optical gas concentration measuring method by forming a differential signal using lights with different absorbabilities to a raw material in a gas flow path using a time-sharing method

Abstract

To provide a concentration measurement method with which the concentrations of predetermined chemical components can be measured non-destructively, accurately, and rapidly by a simple means, up to the concentrations in trace amount ranges, as well as a concentration measurement method with which the concentrations of chemical components in a measurement target can be accurately and rapidly measured in real time up to the concentrations in nano-order trace amount ranges, and which is endowed with a versatility that can be realized in a variety of embodiments and modes. In the present invention, a measurement target is irradiated, in a time sharing manner, with light of a first wavelength and light of a second wavelength that have different optical absorption rates with respect to the measurement target. The light of each wavelength, arriving optically via the measurement target as a result of irradiation with the light of each wavelength, is received at a shared light-receiving sensor. A differential signal is formed, the differential signal being of a signal pertaining to the light of the first wavelength and a signal pertaining to the light of the second wavelength, the signals outputted from the light-receiving sensor upon receipt of the light. The concentration of a chemical component in the measurement target is derived on the basis of the differential signal.

What is claimed is: 
     
       1. An optical gas concentration measuring method when a chemical or physical process is to be implemented upon introduction of a raw material gas into a processing chamber from a gas supply line in which optical gas concentration measurement apparatus comprising a gas flow path is arranged in a predetermined position, the method comprising the steps of:
 irradiating light having a first wavelength that has an absorbability with respect to the raw material gas, and light having a second wavelength that has no or substantially no absorbability with respect to the raw material gas, or an absorbability that is relatively lower than that of the light having the first wavelength, along the same or substantially the same optical path and onto the raw material gas in the gas flow path using a time-sharing method; 
 receiving in a photodetector a first exiting light on the basis of the light having the first wavelength and a second exiting light on the basis of the light having the second wavelength, the first exiting light and the second exiting light exiting from the optical path, in a photodetector disposed on an end edge of the optical path; 
 inputting a first differential circuit input signal in accordance with a first signal based on the first exiting light, and a second differential circuit input signal in accordance with a second signal based on the second exiting light, the first signal and the second signal being output by the photodetector according to the received light, into a differential circuit; and 
 comparing a measured value that is based on an output signal output from the differential circuit in accordance with the input, with data stored in advance in storage means to derive a concentration of the raw material gas, wherein 
 the first signal and the second signal are formed by the photodetector in a first period (T 13 ) that is arranged to be within a second period (T 12 ), 
 the photodetector is in an ON-state within second period (T 12 ), 
 the second period (T 12 ) is arranged to be the same as or within a third period (T 11 ), 
 the light is irradiated onto the raw material gas within third period (T 11 ), and 
 said first, second and third periods satisfy the relationship T 13 <T 12 <T 11 . 
 
     
     
       2. The optical gas concentration measuring method according to  claim 1 , wherein the first signal and the second signal are voltage signals. 
     
     
       3. The optical gas concentration measuring method according to  claim 1 , wherein the first differential circuit input signal and the second differential circuit input signal are voltage signals. 
     
     
       4. An optical gas concentration measuring method when a chemical or physical process is to be implemented upon introduction of a raw material gas into a processing chamber from a gas supply line in which optical gas concentration measurement apparatus comprising a gas flow path is arranged in a predetermined position, the method comprising the steps of:
 irradiating light having the first wavelength and light having the second wavelength, each having a different light absorptivity with respect to the raw material gas, onto the raw material gas in the gas flow path using a time-sharing method; 
 receiving the light of each wavelength that optically passes through the raw material gas according to the irradiation of the light of each wavelength, using a common light-receiving sensor; 
 forming a differential signal between a first signal related to the light having the first wavelength and a second signal related to the light having the second wavelength output from the light-receiving sensor in accordance with the received light; and 
 deriving a concentration of a chemical component of the object to be measured on the basis of the differential signal, wherein 
 the first signal and the second signal are formed by the light-receiving sensor in a first period (T 13 ) that is arranged to be within a second period (T 12 ), 
 the light-receiving sensor is in an ON-state within second period (T 12 ), 
 the second period (T 12 ) is arranged to be the same as or within a third period (T 11 ), 
 the light is irradiated onto the raw material gas within third period (T 11 ), and 
 said first, second and third periods satisfy the relationship T 13 <T 12 <T 11 . 
 
     
     
       5. The optical gas concentration measuring method according to  claim 4 , wherein the first signal and the second signal are voltage signals. 
     
     
       6. The optical gas concentration measuring method according to  claim 5 , wherein the differential signal is a voltage signal.